IBM DS8000 - Manual (Page 210)

188 DS8000 Series: Concepts and Architecture
For example, in the zSeries environment, the RMF™ RAID rank report can be used to
investigate RAID rank saturation when the DS8000 is already installed in your environment.
New counters will be reported by RMF that will provide statistics on a volume level for
channel and disk analysis.
In an open system environment the iostat command is useful to determine whether a
system’s I/O load is balanced or whether a single volume is becoming a performance
bottleneck. The tool reports I/O statistics for TTY devices, disks, and CD-ROMs. It monitors
I/O device throughput and utilization by observing the time the disks are active in relation to
their average transfer rates. The
vmstat utility may also be used to take a quick snapshot or
overview of the system performance.
9.6.8 Hot spot avoidance
Workload activity concentrated on a limited number of RAID ranks will saturate the RAID
ranks. This may result in poor response times, so balancing I/O activity across any disk
system is important. Spreading your I/O activity evenly across the available DS8000s will
enable you to optimally exploit the DS8000 resources, thus providing better performance. I/O
activity attributes to consider include spreading I/O activity across:
The two servers of the DS8000
The loops of the DS8000
All available RAID ranks
Across multiple DS8000s